Method and apparatus for controlling a continuously variable transmission

ABSTRACT

A continuously variable transmission includes a hydrodynamic device, such as a fluid coupling or a torque converter, having a lock-up clutch. The reduction ratio of the continuously variable transmission is controlled by a shift motor. The shift motor is controlled such that each of rotary positions thereof corresponds uniquely to a desired optimum reduction ratio. Upon detecting a state wherein a rapid down shift is needed, the lock-up clutch is temporarily disengaged to allow the hydrodynamic device to slip during the rapid down shift.

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus forcontrolling a continuously variable transmission, and more particularlyto a method and an apparatus for controlling a lock-up clutch of ahydrodynamic device of a continuously variable transmission of anautomotive vehicle.

The term "hydrodynamic device" is used herein to refer to a fluidcoupling and a torque converter.

In a method for controlling a continuously variable V-belt transmissionwhich was previously proposed in pending Japanese Patent Application No.56-44749 filed Mar. 28, 1981 which corresponds to copending U.S. patentapplication Ser. No. 362,489 filed Mar. 26, 1982 and commonly assignedherewith, a lock-up clutch was engaged at vehicle speeds above arelatively low vehicle speed value for better fuel economy byeliminating a power loss due to slip in the hydrodynamic device, and theengagement of the lock-up clutch was released only in a low vehiclespeed operating range in order to prevent the occurrence of unpleasantvibrations at low speeds, and to secure a traction force great enoughfor moving the vehicle from standstill.

However, with the above method, the acceleration performance was notsatisfactory because the hydrodynamic device was maintained in itsengaged condition even when the transmission was to shift down to alarge reduction ratio to meet a rapid acceleration demand. The enginecould not increase its speed promptly for greater torque.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and anapparatus for controlling a continuously variable transmission includinga hydrodynamic device with a lock-up clutch wherein the engine isallowed to increase its speed for greater torque when a rapidacceleration is needed.

More particularly, an object of the present invention is to allow theengine to increase its speed by disengaging temporarily the lock-upclutch upon detecting a state wherein a rapid down shift is needed tomeet a rapid acceleration demand.

According to the present invention, there is provided a method forcontrolling a continuously variable transmission which includes ahydrodynamic device having an input element and an output element, adrive pulley connected to the input element, a driven pulley, a V-beltrunning over the drive and driven pulleys, and a lock-up clutchengageable to mechanically connect the input element with the outputelement, the transmission being shiftable between different reductionratios and having a shift motor which is movable to different positionsfor effecting shifting to diffeent reduction ratios, the methodcomprising:

detecting a state wherein a rapid down shift is needed and generating arapid down shift need indicative signal; and

disengaging the lock-up clutch in response to said rapid down shift needindicative signal so as to allow the hydrodynamic device to slip duringthe down shift.

According to the present invention, there is provided an apparatus forcontrolling a continuously variable transmission of an automotivevehicle having an internal combustion engine, the transmission includinga hydrodynamic device having an input element and an output element, adrive pulley, a driven pulley, a V-belt running over the drive anddriven pulleys, and a lock-up clutch engageable to mechanically connectthe input element to said output element, the transmission beingshiftable between different reduction ratios, the apparatus comprising:

means including a shift motor movable to different positions forshifting the transmission to different reduction ratios correspondinguniquely to said positions;

means for detecting a state wherein a rapid down shift is needed andgenerating a rapid down shift need indicative signal; and

means for disengaging the lock-up clutch in response to said rapid downshift need indicative signal to allow the hydrodynamic device to slipduring the down shift.

According to the present invention, there is provided an apparatus forcontrolling a continuously variable transmission of an automotivevehicle having an internal combustion engine, the transmission includinga hydrodynamic device having an input element and an output element, adrive pulley, a driven pulley, a V-belt running over the drive anddriven pulleys, and a lock-up clutch engageable to mechanically connectthe input element to the output element, the transmission beingshiftable between the reduction ratios, the apparatus comprising:

means for detecting a vehicle speed of the automotive vehicle andgenerating a vehicle speed indicative signal;

means for detecting an engine load on the internal combustion engine;

means for retrieving a desired optimum reduction ratio for the detectedvehicle speed and engine load and generating a desired optimum reductionratio indicative signal;

means for generating an actual reduction ratio indicative signalrepresenting an actual reduction ratio of the continuously variabletransmission;

means for comparing the actual reduction ratio indicative signal withthe desired optimum reduction ratio indicative signal to compute adifference therebetween and generating a difference indicative signalrepresenting said difference;

means for comparing said difference with a predetermined value andgenerating a rapid down shift need indicative signal when saiddifference is greater than said predetermined value;

a shift motor rotatable between a plurality of rotary positions thereof;

means operatively connected to said shift motor to be actuated therebyfor shifting the transmission to a reduction ratio corresponding to oneof the plurality of rotary positions of said shift motor assumed by saidshift motor;

means responsive to said vehicle speed indicative signal for engagingthe lock-up clutch; and means for disengaging the lock-up clutchtemporarily in response to said rapid down shift need indicative signalso as to allow said hydrodynamic device to slip during the rapid downshift.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is more specifically described in connection withthe accompanying drawings, wherein:

FIG. 1 is a diagrammatic cross sectional view illustrating a powertransmission mechanism of a continuously variable V-belt transmission;

FIG. 2 is a layout of the shafts of the transmission mechanism shown inFIG. 1;

FIG. 3 is a view showing a hydraulic control system for the continuouslyvariable V-belt transmission shown in FIGS. 1 and 2;

FIG. 4 is a block diagram showing a control unit for controlling astepper motor 110 and a lock-up solenoid 200 shown in FIG. 3;

FIGS. 5(a) and 5(b) illustrate a flow chart of a lock-up solenoidcontrol routine;

FIG. 6 is a diagrammatic view illustrating how lock-up on vehicle speeddata are stored in a ROM 314 shown in FIG. 4;

FIG. 7 is a flow chart showing a data retrieval routine for lock-up onvehicle speed data;

FIG. 8 is a graph showing the relationship between lock-up vehicle speedVon and lock-up off vehicle speed Voff;

FIGS. 9(a) and 9(b) illustrate a flow chart showing a stepper motorcontrol routine;

FIG. 10 is a flow chart showing a D range pattern data retrievalroutine;

FIG. 11 is a diagrammatic view illustrating how pulse number data ND arestored in a matrix in the ROM 314 versus throttle opening degree andvehicle speed;

FIG. 12 is a chart illustrating various modes of stepper motor actuatingsignals applied to output leads 317a, 317c, 317b and 317d of the steppermotor 110;

FIG. 13 is a diagrammatic view of the content of four bit positionscorresponding to the mode A;

FIG. 14 is a timing diagram of the stepper motor actuating signals;

FIG. 15 is a graph showing a minimum fuel consumption rate curve G;

FIG. 16 is a graph showing the minimum fuel consumption rate curveexpressed in terms of the throttle opening degree and engine revolutionspeed;

FIG. 17 is a graph showing the relationship shown in FIG. 16 expressedin terms of the throttle opening degree and vehicle speed for variousreduction ratios;

FIG. 18 is a graph showing a predetermined relationship of the reductionratio with the stepper motor pulse number;

FIG. 19 is a graph showing a predetermined relationship shown in FIG. 16expressed in terms of the throttle opening degree and vehicle speed forvarious pulse numbers;

FIG. 20 is a graph showing the minimum fuel consumption rate curveexpressed in terms of intake manifold vacuum and engine revolutionspeed;

FIG. 21 is a graph showing the minimum fuel consumption rate curveexpressed in terms of fuel flow rate;

FIGS. 22(a) and 22(b) illustrate a flow chart of a lock-up controlroutine of a second embodiment according to the present invention; and

FIGS. 23(a) and 23(b) illustrate a flow chart of a lock-up controlroutine of a third embodiment according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the accompanying drawings, the present invention isdescribed hereinafter in connection with preferred embodiments.

Referring to FIG. 1, a torque converter 12 (which may be replaced with afluid coupling) includes a pump impeller 4, a turbine runner 6, a stator8 and a lock-up clutch (lock-up device) 10. The lock-up clutch 10 isconnected to the turbine runner 6 in an axially slidable manner andcooperates with a member (converter shell) 4a coupled with the engineoutput shaft 2 integral with the pump impeller 4 to define a lock-upclutch oil chamber 14. The lock-up clutch 10 operates such that when theoil pressure within the lock-up clutch oil chamber 14 becomes lower thanthat within the interior of the torque converter 12, the lock-up clutch10 is pressed against the member 4a by the pressure difference to comeinto a unitary rotation therewith. The turbine runner 6 is splined toone end of a drive shaft 22 which is rotatably supported by a case 20via bearings 16 and 18. Arranged on the drive shaft 22 between thebearings 16 and 18 is a drive pulley 24. The drive pulley 24 comprisesan axially fixed conical disc 26 secured to the drive shaft 22 and anaxially movable conical disc 30 which is so disposed as to face theaxially fixed conical disc 26 to define a V-shaped pulley groovetherebetween and which is controllably movable in an axial direction ofthe driven shaft 22 in response to an oil pressure created within adrive pulley cylinder chamber 28 (see FIG. 3). For limiting the maximumwidth of the V-shaped pulley groove, an annular member 22a is fixed tothe drive shaft 22 and so disposed as to engage the driven pulley 34(see FIG. 3). The drive pulley 24 is drivingly connected to a drivenpulley 34 via a V-belt 32. The V-belt 32 runs over the drive pulley 24and the driven pulley 34. The driven pulley 34 is arranged on a drivenshaft 40 which is rotatably supported by the case 20 via the bearings 36and 38. The driven pulley 34 comprises an axially fixed conical disc 42secured to the driven shaft 40 and an axially movable conical disc 46which is so disposed as to face the fixed conical disc 42 in such amanner as to define a V-shaped pulley groove and which is controllablymovable in an axial direction of the drive shaft 40 in response to anoil pressure created in a driven pulley cylinder chamber 44 (see FIG.3). Similarly to the drive pulley 24, an annular member 40a is fixed tothe driven shaft 40 to limit the movement of the axially slidableconical disc 46 so as to define the maximum width of the V-shaped pulleygroove. The aixally fixed conical disc 42 is drivingly connectable via aforward drive multiple disc clutch 48 to a forward drive gear 50rotatably supported on the driven shaft 40, this forward drive gearbeing in mesh with a ring gear 52. Fixedly mounted to the driven shaft40 is a reverse drive gear 54 which is in mesh with an idler gear 56.The idler gear 56 is drivingly connectable via a reverse drive multipledisc clutch 58 to an idler shaft 60 which has fixed thereto anotheridler gear 62 that is in mesh with the ring gear 52. In FIG. 1, theidler gear 62, idler shaft 60 and reverse drive multiple disc clutch 54are illustrated in positions displaced from the actual positions thereoffor ease of illustration, the idler gear 62 and ring gear 52 are shownas out of mesh with each other. But, they are actually in mesh with eachother as shown in FIG. 2. The ring gear 52 has attached thereto a pairof pinion gears 64 and 66. A pair of axle or output shafts 72 and 74 arecoupled with side gears 68 and 70, respectively, which are in mesh withthe pinion gears 64 and 66 to cooperate to form a differential 67. Theaxle shafts 72 and 74 are supported via bearings 76 and 78,respectively, extend outwardly of the case 20 in the opposite directionsand are connected to road wheels (not shown), respectively. As viewed inFIG. 1, there is arranged on the righthand side of the bearing 18 an oilpump 80 of the internally toothed gearing type which serves as a sourceof oil pressure, this oil pump 80 being driven by the engine outputshaft 2 via an oil pump drive shaft 82 extending through the hollowdriven shaft 22.

Rotary power fed from the engine output shaft 2 is transmitted via thetorque converter 12, drive shaft 22, drive pulley 24, V-belt 32, drivenpulley 34 to driven shaft 40 and in the case the forward multiple discclutch 48 is engaged, with the reverse drive multiple disc clutch 58released, the rotation of the shaft 40 is transmitted via the forwarddrive gear 50, ring gear 52 and differential 67 to the axle shafts 72and 74 to rotate them in the forward rotational direction, whereas, inthe case the reverse drive multiple disc clutch 58 is engaged with theforward drive multiple disc clutch 48 released, the rotation of theshaft 40 is transmitted via the reverse drive gear 54, idler gear 56,idler shaft 60, idler gear 62, ring gear 52 and differential 67 to theaxle shafts 72 and 74 to rotate them in the reverse rotationaldirection. During this transmission of power, the ratio between therotation of the drive pulley 24 and that of the driven pulley 34 may bevaried by moving the axially movable conical disc 30 of the drive pulley24 and the axially movable conical disc 46 of the driven pulley 34 in anaxial direction so as to change the radii of the diameter contactingwith the V-belt 32. For example, increasing the width of the V-shapedpulley groove of the drive pulley 24 and decreasing the width of theV-shaped pulley groove of the driven pulley 34 will result in areduction in radius of the diameter of the drive pulley 24 contactingwith the V-belt 32 and an increase in radius of the diameter of thedriven pulley 34 contacting with the V-belt 32, resulting in an increasein reduction ratio. As a result, a reduction ratio increases. If theaxially movable conical discs 30 and 46 are moved in the respectivereverse directions, a reduction ratio decreases. Depending upon thesituations, the torque converter serves as a torque multiplier or servesas a fluid coupling but, since it has the lock-up clutch 10 as attachedto the turbine runner 6, the torque converter 12 can establish a directmechanical connection between the engine output shaft 2 and driven shaft22 when the lock-up clutch oil chamber 14 is drained, thus pressing thelock-up clutch 10 against the member 4a integral with the pump impeller4.

Referring to FIG. 3, a hydraulic control system for the continuouslyvariable transmission is explained. As shown in FIG. 3, the controlsystem comprises an oil pump 80, a line pressure regulator valve 102, amanual valve 104, a shift control valve 106, a lock-up valve 108, ashift motor 110, and a shift operating mechanism 112.

The oil pump 80 which is driven by the engine output shaft 2 draws offthe oil from the tank 114 and discharges the oil under pressure into theoil conduit 116. The oil conduit 116 leads to ports 118d, 118f and 118gof the line pressure regulator valve 102 where the oil is regulated togenerate a pressure oil under line pressure. This pressure oil ishereinafter referred to as a line pressure. The oil conduit 116communicates with a port 120b of the manual valve 104 and a port 122c ofthe shift control valve 106.

The manual valve 104 has a valve bore 120 with five ports 120a, 120b,120c, 120d and 120e, and a spool 124 having thereon two lands 124a and124b slidably disposed in the valve bore 120. The spool 124 is actuatedmanually by a shift lever (not shown) between five detent positions orshift positions for P range, R range, N range, D range and L range. Theport 120a is permitted to communicate not only with a port 120d via anoil conduit 126, but also with a cylinder chamber 58a of the reversedrive multiple disc clutch 58. A port 120c is permitted to communicatenot only with a port 120e by an oil conduit 130, but also with acylinder chamber 48a of a forward drive multiple disc clutch 48. Theport 120b communicates with the oil conduit 116 to receive the linepressure therein. When the spool 124 is set in P range, the port 120bsupplied with the line pressure is covered by a land 124b, so that thecylinder chamber 58a of the reverse drive multiple disc clutch 58 andthe cylinder chamber 48a of the forward drive multiple disc clutch 48are drained via the oil conduit 126 and ports 120d and 120e. When thespool 124 is set in R range, the port 120b is permitted to communicatewith the port 120a by a groove between the lands 124a and 124b so as topermit the line pressure to communicate with the cylinder chamber 58afor the reverse drive multiple disc clutch 58, whereas, the cylinderchamber 48a of the forward drive multiple disc clutch 48 is left drainedvia the port 120e. When the spool 124 is set in N range, the port 120bis disposed between the lands 124a and 124b and is prevented fromcommunicating with the other ports, thus the cylinder chamber 58a of thereverse drive multiple disc clutch 58 and the cylinder chamber 48a ofthe forward drive multiple disc clutch 48 are drained via the port 120aand port 120e in a similar manner to the case when the spool is set in Prange. When the spool 124 is set in D range or L range, the port 120b ispermitted to communicate with the port 120c via the groove between theport 120b and 120c so that the line pressure is supplied to the cylinderchamber 48a of the forward multiple disc clutch 48, whereas, thecylinder chamber 58a of the reverse drive clutch 58 is drained via theport 120a. Therefore, when the spool 124 is set in P range or N range,both the forward drive multiple disc clutch 48 and the reverse drivemultiple disc clutch 58 are released to interrupt the transmission ofpower, thus preventing the rotation of output shatfs 72 and 74. When thespool 124 is set in R range, the reverse drive multiple disc clutch 58is engaged so as to drive the axle shafts 72 and 74 in the reverserotational direction. When the spool 124 is set in D range or L range,the forward drive multiple disc clutch 48 is engaged so as to drive theaxle shafts 72 and 74 in the forward rotational direction. Althoughthere occurs no difference in the respect of a hydraulic circuit betweenthe case where D range is selected and the case where L range isselected as mentioned above, both of the ranges are electricallydetected to actuate the shift motor 110 in such a manner as to effect ashift control in accordance with different shift patterns.

The line pressure regulator valve 102 comprises a valve bore 118 witheight ports 118a, 118b, 118c, 118d, 118e, 118f, 118g and 118h; a spool132 having thereon four lands 132a, 132b, 132c, and 132d, and a spring133c disposed on the lefthand side of the spool 132; and a spring seat134 fixed relative to the valve bore 118 by a pin 135. It is to be notedthat the land 132d on the righthand end of the spool 132 is smaller indiameter than the middle lands 132a, 132b and 132c. A vacuum diaphragm143 is arranged on the inlet of the bore 118. The vacuum diaphragm 143is constructed of two parts 136a and 136b which clamp therebetween adiaphragm 137 and cooperate with each other to form a casing 136. Thecasing 136 is divided by the diaphragm 137 into two chambers 139a and139b. Attached by a fixer 137a to the diaphragm 137 is a spring seat137b with which a spring 140 is disposed in the chamber 139a biasing thediaphragm 137 to the right. The intake manifold vacuum is introducedinto the chamber 139a via a port 142, while the other chamber 139b isvented to the atmosphere via a port 138. Arranged between the diaphragm137 of the vacuum diaphragm 143 and the spool 132 is a rod 141 extendingthrough the spring seat 134 so as to apply a rightwardly directed biasforce to the spool 132. The arrangement is such that this bias forceincreases as the intake manifold vacuum decreases or becomes small. Thatis, if the intake manifold vacuum is small (i.e., if the intake manifoldvacuum is near the atmospheric pressure), a large rightwardly directedforce by the spring 140 is applied to the spool 132 through the rod 141since a difference in pressure between the chambers 139a and 139b issmall and thus the leftwardly directed force caused by this pressuredifference and applied to the diaphragm 137 is small. In the reversecase where the intake manifold vacuum is large, the force applied to thespool 132 becomes small since the leftwardly directed force caused bythe pressure difference between the chambers 139a and 139b becomes largeand thus the rightwardly directed force by the spring 140 decreasescorrespondingly. The ports 118d, 118f and 118g of the line pressureregulator valve 102 are supplied with the oil under pressure from theoil pump 80, and the inlet to the port 118g is provided with an orifice149. The ports 118a, 118c and 118h are at all times drained, and theport 118e is connected by an oil conduit 144 with the torque converterinlet port 146 and also with the ports 150c and 150d of the lock-upvalve 108, and the port 118b is connected by an oil conduit 148 with theport 150b of the lock-up valve 108 and also with the lock-up clutch oilchamber 14. For preventing the application of an excessive pressure tothe interior of the torque converter 12, the oil conduit 144 is providedwith an orifice 145. Consequently, three forces act on the spool 132 inthe rightward direction, i.e., one by the spring 133, another by thevacuum diaphragm 143 via the rod 141 and the other by the oil pressureapplied to the left end of the land 132a via the port 118b. One forceacts on the spool 132 in the leftward direction by the line pressure atthe port 118g acting on differential area between the lands 132c and132d. The spool 132 effects pressure regulation to provide the linepressure at the port 118d by adjusting the amount of drainage oilpassing from the ports 118f and 118d to the respective ports 118e and118c (i.e., first of all the oil is drained from the port 118f to theport 118e and, if more drainage is demanded, the oil is drained from theport 118d to the port 118c) until the rightwardly directed forcesbalance with the leftwardly directed force. As a result, the linepressure increases as the engine intake manifold vacuum drops and itincreases as the oil pressure building up in the port 118b (i.e., thesame pressure as in the lock-up clutch oil chamber 14) increases.Because the oil (in this case pressure exists in the oil chamber 14, thetorque converter 12 is in a non lock-up state and serves as a torquemultiplier. The variation in the line pressure in this manner meets theactual demands, i.e., the line pressure must be increased to increase abracing force with which each of the pulleys 24 and 34 are biasedagainst the V-belt 32 in response to an increase in the torque to betransmitted via the pulleys which increases as the engine loadincreases, i.e., as the intake manifold vacuum decreases, and besidesthe line pressure must be increased to increase the torque to betransmitted via the pulley as the multiplication of torque by the torqueconverter 12 increases.

The shift control valve 106 has a valve bore 122 with five ports 122a,122b, 122c, 122d and 122e, and a spool 152 slidably disposed in thevalve bore 122 and having thereon four lands 152a, 152b, 152c and 152d.The center port 122c communicates with the oil conduit 116 and issupplied with the line pressure, the left port 122b and the right port122d communicate via respective conduits 154 and 156 with the drivepulley cylinder chamber 28 of the drive pulley 24 and the driven pulleycylinder chamber 44 of the driven pulley 34. Both of the end ports 122aand 122e are drained. The left end of the spool 152 is linked to asubstantially middle portion of a lever 160 of the later-mentioned shiftoperating mechanism 112. The width of each of the lands 152b and 152c isset slightly shorter than the width of the respective ports 122b and122d, and the distance between the lands 152b and 152c is setsubstantially the same as that between the ports 122b and 122d.Therefore, a portion of the line pressure supplied via the port 122c tothe oil chamber between the lands 152b and 152c is allowed to passthrough a clearance formed between the land 152b and the port 122b toflow into an oil conduit 154, but the remaining portion thereof isallowed to pass through another clearance formed between the land 152band the port 122b to be drained, so that the pressure within the oilconduit 154 is determined depending upon the ratio between the areas ofthe above-mentioned clearances. In a similar manner, the pressure withinthe oil conduit 156 is determined depending upon the ratio of the areasof clearances formed between the edges of the land 152c and the port122d. Therefore, if the spool 152 is disposed in the center position,the relationship of the land 152b with the port 122b becomes equal tothat of the land 152c with the port 122d, thus causing the pressure inthe oil conduit 154 to become equal to that in the oil conduit 156. Asthe spool 152 moves leftwardly, the clearance of the port 122b on theline pressure side increases and the clearance thereof on the drain sidedecreases, thus allowing the pressure in the oil conduit 154 to increaseaccordingly, whereas, the clearance of the port 122d on the linepressure side decreases and the clearance thereof on the drain sideincreases, thus causing the pressure in the oil conduit 156 to decreaseaccordingly. This causes an increase in pressure in the drive pulleycylinder chamber 28 of the drive pulley 24, resulting in a decrease inthe width of the V-shaped pulley groove, and a reduction in pressure inthe driven pulley cylinder chamber 44 of the driven pulley 34, resultingin an increase in the width of the V-shaped pulley groove, so thatbecause the radius of the diameter of the drive pulley 24 contactingwith the V-belt increases and the radius of the diameter of the drivenpulley 34 contacting with the V-belt decreases, a reduction ratiodecreases. If the spool 152 is urged to move rightwardly, the reverseaction to that mentioned above proceeds to cause an increase in thereduction ratio.

The lever 160 of the shift operating mechanism 112, which lever is pinconnected at its middle portion with the spool 152 of the shift controlvalve 106, has its one end received in an annular groove 30a formed inthe axially movable conical disc 30 of the drive pulley 24 and has itsopposite end pin connected with the sleeve 162. The sleeve 162 isinternally threaded to mesh with the thread formed on the shaft 168which is rotatable by the shift motor 110 via the gears 164 and 166.With this shift operating mechanism 112, if the shift motor 110 isrotated to rotate the shaft 168 via the gears 164 and 166 in onerotatioal direction to cause the sleeve 162 to move leftwardly, thelever 160 moves in a clockwise rotational direction with its end portionreceived by the annular groove 30a of the axially movable conical disc30 of the drive pulley 24 as an fulcurum point, causing the leftwardmovement of the spool 152 connected to the lever 160 of the shiftcontrol valve 106. This causes a rightward movement of the axiallymovable conical disc 30 of the drive pulley 24 in a manner mentionedbefore to decrease the width of the V-shaped pulley groove, while, atthe same time, the width of the V-shaped pulley groove of the drivenpulley 34 increases, thus resulting in a decrease in the reductionratio. Since the one end of the lever 160 is engaged with the groove 30aaround the outer periphery of the axially movable conical disc 30,urging the axially movable conical disc 30 to move rightwardly willrotate the lever 160 clockwise with that end of the lever 160 which ispin connected with the sleeve 162 as a fulcrum. This causes the spool152 to move back rightwardly, tending to render the drive pulley 24 anddriven pulley 34 to assume the state accomplishing a large reductionratio. This action causes the spool 152 and the drive pulley 24 anddriven pulley 34 to assume a state accomplishing a reduction ratiodepending upon the amount of rotation of the shift motor 110. It goesthe same if the shift motor 110 is rotated in the reverse direction.Therefore, if the shift motor 110 is actuated in accordance with apredetermined shift pattern, the reduction ratio varies accordingly,thus making it possible to control the reduction ratio in thecontinuously variable transmission by controlling the shift motor 110,alone.

The shift motor 110 is controlled by a control unit 300 which isdescribed later in more detail in connection with FIG. 4.

The lock-up valve 108 comprises a valve bore 150 formed with four ports150a, 150b, 150c and 150d, a spool 170 having two lands 170a and 170b, aspring 172 biasing the spool 170 rightwardly, and a lock-up solenoid 200provided in the oil conduit communicating with the port 150d. The port150a is drained. The port 150b communicates via an oil conduit 148 withthe port 118b of the line pressure regulator valve 102 and also with thelock-up clutch oil chamber 14 within the torque converter 12. The ports150c and 150d are connected with each other via an orifice 201. A branchoil conduit 207 is formed between the port 150d and the orifice 201. Thebranch oil conduit 207 opens via an orifice 203 and has its outlet to beclosed or opened by the lock-up solenoid 200 in response to on statethereof or off state thereof. The size of the orifice 203 is greaterthan that of the orifice 201. When the lock-up solenoid 200 is in the onstate, the spool 170 is pressed against the force of the spring 172toward the left because the same oil pressure as that supplied to thetorque converter inlet port 146 is supplied to the port 150d from theoil conduit 144 as a result of closing of the outlet of the branch oilconduit 207. In this state, the port 150c is blocked by the land 170band the port 150b is allowed to drain toward the port 150a. As a result,the lock-up clutch oil chamber 14 which has been connected with the oilpressure via the port 150b and the oil conduit 148 is drained, allowingthe lock-up clutch 10 to be engaged under the influence of the pressurein the torque converter 12, thus putting the torque converter 12 intolock-up state where the torque converter does not serve as a torqueconverter. In the reverse case when the lock-up solenoid 200 is put intothe off state, the spool 170 is moved in the rightward direction by therightwardly directed force by the spring 172 and the port 150b isallowed to communicate with the port 150c since the oil pressure at theport 150d drops due to uncovering of the outlet of the branch oilconduit 207 (the portion of the oil conduit 144 which is subjected tothis drop in pressure is confined to a portion between the orifice 201and the port 150d leaving the remainder of the oil conduit 144unaffected to this pressure drop owing to the provision of the orifice201) and this causes the force biasing the spool 170 to the left todisappear. As a result, the oil conduit 148 is allowed to communicatewith the oil conduit 144, applying the same oil pressure as that appliedto the torque converter inlet port 146 to the lock-up clutch oil chamber14, causing the pressures on the both sides of the lock-up clutch 10 tobecome equal to each other, resulting in the release of the lock-upclutch 10. An orifice 174 is provided in the inlet of the port 150c andanother orifice 178 is provided in the drain oil conduit connected withthe port 150a. The orifice 178 is provided to prevent rapid drainage ofthe oil pressure from the lock-up clutch oil chamber 14 so as toalleviate a shock upon shifting into the lock-up state, whereas, theorifice 174 is provided in the oil conduit 144 to permit a gradualincrease in oil pressure within the lock-up oil chamber 14 so as toalleviate a shock upon release from the lock-up state.

The torque converter outlet port 180 communicates with the oil conduit182 which is provided with a relief valve 188 including a ball 184 and aspring 186 and thus, with this relief valve, the pressure within thetorque converter 12 is maintained within normal operating pressurerange. The oil downstream of the relief valve 188 is introduced by anoil conduit 190 to an oil cooler and a lubricant circuit, both beingunillustrated, and is finally drained, whereas, an excessive oil isdrained by another relief valve 192, the thus drained oil being returnedfinally to a tank 114.

Next, an explanation is made regarding the control unit 300 whichcontrols the shift motor 110 and the lock-up solenoid 200. The shiftmotor 110 is a stepper motor and thus referred hereinafter to as thestepper motor.

As shown in FIG. 4, the control unit 300 receives input signals from anengine revolution speed sensor 301, a vehicle speed sensor 302, athrottle opening degree sensor 303, a shift position switch 304, a shiftreference switch 240, an engine coolant temperature sensor 306, and abrake sensor 307. The engine revolution speed sensor 301 detects anengine revolution speed by measuring the number of ignition spark plusesof the engine per unit time, and the vehicle speed sensor 302 detects avehicle speed by measuring the revolution of the output shaft of thecontinuously variable transmission. The throttle opening degree sensor303 detects the engine load by measuring the engine throttle openingdegree, and generates an electric voltage signal. The throttle openingdegree sensor 303 may be replaced with an intake manifold vacuum sensoror a fuel flow rate sensor. The shift position switch 304 detects whichone of the range positions, namely, P range, N range, D range, and Lrange, is selected by the manual valve 104. The shift reference switch240 is turned on when the sleeve 162 of the shift operating mechanism112 assumes a position corresponding to the largest reduction ratio. Forthis purpose, the shift reference switch 240 is disposed such that it isturned on when the sleeve 162 is moved to the rightward limit positionviewing in FIG. 3. The engine coolant temperature sensor 306 generates asignal when the engine coolant temperature is lower than a predeterminedvalue. The brake sensor 307 detects whether or not the vehicle brake isactuated. The sensor output signals generated by the engine revolutionspeed sensor 301 and vehicle speed sensor 302 are sent to an inputinterface 311 after passage through wave shapers 308 and 309,respectively. The electric voltage from the throttle opening degreesensor 303 is converted by an analog-digital (A/D) converter 310 into adigital signal before being sent to the input interface 311. In additionto the input interface the shift control unit 300 comprises a referencepulse generator 312, a CPU (Central Processor Unit) 313, a ROM (ReadOnly Memory) 314, a RAM (Random Access Memory) 315, and an outputinterface 316, which are linked with each other by an address bus 319and a data bus 320. The reference pulse generator 312 generatesreference pulses with which the CPU 313 is actuated. The ROM 314 storesprograms necessary for controlling the stepper motor 110 and lock-upsolenoid 200 and data necessary for controlling them. The RAM storesvarious parameters necessary for processing information from each of thesensors and switches and parameters necessary for controlling thestepper motor 110 and lock-up solenoid 200. Output signals from thecontrol unit 300 are sent to the stepper motor 110 and lock-up solenoid200 via respective amplifiers 317 and 318.

Hereinafter, a concrete explanation is made regarding a control methodcarried out by this control unit 300 in controlling the stepper motor110 and lock-up solenoid 200.

The control may be divided into two routines, one being a lock-upsolenoid control routine 500, the other being a stepper motor controlroutine 700.

First of all, the control of the lock-up solenoid 200 is explained. Thelock-up solenoid control routine 500 is shown in FIGS. 5(a) and 5(b).The lock-up solenoid control routine 500 is executed once per apredetermined period of time. Thus, the execution of the followingroutine is repeated after a short period of time. A throttle openingdegree signal TH indicative of the engine load is obtained from thethrottle opening degree sensor 303 in step 501, then a vehicle speedindicative signal V is obtained from the vehicle speed sensor 302 instep 503 and after that a shift position indicative signal is obtainedfrom the shift position switch 304 in step 505. A determination is madein step 507 whether any one of the P range, N range and R range isselected, and if the determination result reveals that the P range or Nrange or R range is selected, the lock-up solenoid 200 is deactuated(off state) in step 567 and then, in step 569, the present state of thelock-up solenoid 200 is stored in terms of a lock-up solenoid operatingstate data in the RAM 315 before the program returns to START of theroutine 500. It will now be understood that as long as the P range or Nrange or R range is selected, the lock-up solenoid 200 is not energizedand thus the torque converter 12 is in the non lock-up state. If thedetermination made in the step 507 shows that none of P, N nor R rangeposition is selected, a determination is made whether D range isselected or not in step 1707. If the D range is selected, a D rangeshift pattern retrieval routine is executed in step 1720. The D rangeshift pattern retrieval routine 1720 provides a pulse number ND for thestepper motor which corresponds to the present throttle opening degreeTH and vehicle speed V, but since it is quite the same as a laterdescribed D range shift pattern retrieval routine 720 (FIG. 10), thedetailed description thereof is not made here. If, in the step 1707, theD range is not selected, a L range shift pattern retrieval routine isexecuted in step 1740. The L range shift pattern retrieval routine 1740is basically the same as the D range shift pattern retrieval routine1720 but the data stored therein are different pulse numbers (thisdifference being described later). After retrieving a desired pulsenumber ND for the stepper motor in step 1720 or 1740 (although the pulsenumber data retrieved in step 1740 is NL, it is also represented by NDin the following description), the program goes to a step 1503 whereinan actual pulse number NA is obtained which represents the presentposition of the stepper motor. Then, a difference ΔS between the actualpulse number NA and the desired pulse number ND is computed bysubtracting ND from NA in step 1505 and a determination is made whetherΔS is greater than or equal to a predetermined positive value S1 or notin step 1507. If the difference ΔS is greater than or equal to S1 (i.e.,a state wherein a rapid increase in reduction ratio is needed becausethe actual pulse number NA is considerably greater than the desiredpulse number ND), the program goes to a step 567 to deactuate thelock-up solenoid 200. As a result, the engagement of the lock-up clutchis released. If the difference ΔS is less than S1, the lock-up solenoidoperating state data (whether being actuated or not actuated) in thepreceding routine is obtained from the corresponding address in the RAM315 in step 509. Then, a determination is made in step 511 whether thelock-up solenoid 200 was actuated (in the on state) or not in thepreceding routine. If in the preceding routine the lock-up solenoid 200was not actuated or was in the off state, the data are retrieved in step520 relating to a vehicle speed value (a lock-up on vehicle speed valueVon) above which the lock-up solenoid 200 is to be actuated. The dataretrieval routine 520 is described in connection with FIGS. 6, 7 and 8.Lock-up on vehicle speed data Von, such as, Von1˜Von6, are stored in theROM 314 for the throttle opening degrees as shown in FIG. 6. Referringto FIG. 7, in the data retrieval routine 520, a reference throttleopening degree TH* is given a zero value in step 521 which correspondsto idle state and then an address i for the ROM 314 is given a number i1corresponding to the number zero of the reference throttle openingdegree TH* (in step 522). Then, the actual throttle opening degree TH iscompared with the reference throttle opening degree TH* (in step 523).If the actual throttle opening degree TH is smaller than or equal to thereference throttle opening degree TH*, the number i1 gives an address inthe ROM 314 where an optimum lock-up on vehicle speed data Voncorresponding to the actual throttle opening degree TH is stored and thelock-up on vehicle speed data Von is obtained from the address given bythe number i1 (in step 526). On the other hand, if the actual throttleopening degree TH is greater than the reference throttle opening degreeTH* in the step 523, the reference throttle opening degree TH* isincreased by a predetermined amount ΔTH* (in step 524) and address i isincreased by a predetermined amount Δi (in step 525). Thereafter, theprogram returns to the step 523 again where the actual throttle openingdegree TH is compared with the reference throttle opening degree TH*. Byrepeating this chain of steps (steps 523, 524 and 525), the number ofthe address i in the ROM 314 is given where a lock-up on vehicle speeddata Von corresponding to the actual throttle opening degree TH isstored. Then, the lock-up on vehicle speed data Von is obtained from theaddress i.

Referring again to FIGS. 5(a) and 5(b), after the completion of the dataretrieval routine 520 the lock-up on vehicle speed data Von is comparedwith the actual vehicle speed V (in step 561) and if the actual vehiclespeed V is higher than or equal to the lock-up on vehicle speed Von,then the lock-up solenoid 200 is actuated (in step 563), whereas if therelationship is reversed the lock-up solenoid 200 is not actuated (instep 567), and then operating state data indicating actuating state ordeactuating state is stored in the RAM 315 (in step 569).

If, in the step 511, the lock-up solenoid 200 was found to be actuatedin the preceding routine, a retrieval routine for vehicle speed dataVoff (a lock-up off vehicle speed) below which the lock-up is to bereleased is executed (in step 540). This data retrieval routine 540 issubstantially the same in operation as the data retrieval routine 520for lock-up on vehicle speed data Von (only difference being in thestored data as follows) and therefore the explanation thereof isomitted.

The lock-up on vehicle speed data Von and the lock-up off vehicle speeddata Voff have the relationship as shown in FIG. 8. The relationshipVon>Voff provides a hysterisis. This prevents the occurrence of huntingof the lock-up solenoid 200.

After the completion of the step 540, the lock-up off vehicle speed dataVoff which has been retrieved in the step 540 is compared with theactual vehicle speed V in step 565, and if the vehicle speed V is higherthan or equal to the lock-up off vehicle speed Voff, the lock-upsolenoid 200 is actuated in step 563. If V is lower than Voff, thelock-up solenoid 200 is deactuated in step 567. Then the lock-upsolenoid operating state indicative data is stored in the RAM 315 beforethe program returns to START.

As will now be understood from the above description that, if the D or Lrange is selected, the torque converter 12 is rendered to operate in thelock-up state when the vehicle speed is higher than the lock-up onvehicle speed, whereas it is rendered to operate in the non-lock statewhen the vehicle speed is lower than the lock-up off vehicle speed Voff,and the non lock-up state is maintained irrespective of the actualvehicle speed whenever a rapid down shifting is needed. This means thatwhenever a rapid down shifting is needed (i.e., a state wherein a rapidacceleration is needed) at a vehicle speed higher than the lock-up offvehicle speed Voff, the torque converter is rendered to operate in thenon lock-up state, thus providing a strong and rapid acceleration owingto the torque converter function thereof, thus improving a response ofthe continuously variable V-belt transmission to shifting demand.

Hereinafter, the stepper motor control routine 700 for the stepper motor110 will be explained in connection with FIGS. 9(a) and 9(b). Thestepper motor control routine 700 is executed once per a predeterminedperiod time. Thus, the execution of the following routine is repeatedafter a short period of time. First, the solenoid operating state datawhich was stored in the step 569 (see FIG. 5) of the lock-up solenoidcontrol routine 500 is obtained in step 698 (see FIG. 9(a)), and adetermination is made of the lock-up state in step 699. If the lock-upsolenoid 200 was actuated, the execution of a routine beginning with astep 701 starts, whereas if the lock-up solenoid 200 was not actuated,the execution of a chain of steps beginning with step 713 (see FIG.9(b)) starts. In the latter case the control is made, in a mannerdescribed later, such that the largest reduction ratio is maintained.That is, the largest reduction ratio is maintained during operation withthe torque converter 12 in the non lock-up state.

If, in step 699, the lock-up solenoid 200 is actuated, the throttleopening degree TH is obtained from the throttle opening degree sensor303 in step 701, then the vehicle speed V is obtained from the vehiclespeed sensor 302 in step 703, and after that the shift position isobtained from the shift position switch 304 (in step 705). Subsequently,a determination is made whether the present shift position is the Drange in step 707. If the present shift position is the D range, a Drange shift pattern data retrieval routine is executed in step 720.

The D range shift pattern data retrieval routine in step 720 provides adesired optimum reduction ratio indicative signal. The desired reductionratio indicative signal represents a desired optimum reduction ratio forthe detected operating condition of the automotive vehicle and isexpressed in terms of a number of pulses ND which is hereinafter calledas a stepper motor pulse number. The D range shift pattern dataretrieval routine is executed in a manner illustrated in FIG. 10. Thestepper motor pulse number data ND are stored in the ROM 314 in a matrixshown in FIG. 11. The vehicle speed values are arranged along thelateral axis and the throttle opening degree values are arranged alongthe vertical axis (the vehicle speed increases toward the right in FIG.11 and the throttle opening degree increases toward the bottom in FIG.11). Referring to the D range shift pattern data retrieval routine 720shown in FIG. 10, a reference throttle opening degree TH' is given azero value which corresponds to idle state in step 721 and an address jof the ROM 314 where a stepper motor pulse number data which correspondsto zero throttle opening degree is given a number j' in step 722.Subsequently, the actual throttle opening degree TH is compared with thereference throttle opening degree TH' in step 723. If the actualthrottle opening degree TH is greater than TH', the reference throttleopening degree TH' is increased by ΔTH' in step 724 and the address j isincreased by a predetermined amount Δj in step 725. After this step, theactual throttle opening degree TH is compared with the referencethrottle opening degree TH' again (in step 723), and if the actualthrottle opening degree TH stays greater than TH', the steps 724, 725and 723 are repeated. After the execution of the steps 723, 724 and 725has been repeated, the number j corresponding to the actual throttleopening degree TH is given when the actual throttle opening degree THbecomes equal or smaller than the reference throttle opening degree TH'.Subsequently, steps 726, 727, 728, 729 and 730 are executed in relationto vehicle speed V. As a result, the number k is given which correspondsto the actual vehicle speed V. Then, the number k thus given is combinedwith the number j in step 731, thus producing an address correspondingto a set of the actual throttle opening degree TH and the actual vehiclespeed V, and the stepper motor pulse number data ND is obtained fromthis address in step 732. The pulse number data ND thus obtained shows adesired stepper motor pulse number to be given for the actual throttleopening degree TH and the actual vehicle speed V. The D range shiftpattern data retrieval routine 720 ends with the step of retrieving thepulse number data ND before the program returns to START.

Referring to FIG. 9(a), if the D range is not selected as the result ofthe determination in the step 707, then a determination is made whetherthe L range is selected in step 709, and if the L range is selected, a Lrange shift pattern data retrieval routine is executed (in step 740).The L range shift pattern data retrieval routine is substantiallysimilar to the D range shift pattern data retrieval routine 720 exceptthat the stepper motor pulse number data NL are different from thestepper motor pulse number data ND (the difference between the pulsenumber data ND and NL will be described hereinafter) and are stored atdifferent addresses in the ROM 314. A detailed explanation thereof is,therefore, omitted.

If neither the D range nor the L range is selected, a determination ismade whether the R range is selected in step 711. If the R range isselected, a R range shift pattern data retrieval routine 760 is executedin step 760. The R range shift pattern data retrieval routine 760 issubstantially similar to the D range shift pattern data retrievalroutine 720 except that different stepper motor pulse number data NR arestored and thus a detailed explanation thereof is omitted.

After the data retrieval of the suitable pulse number data ND, NL or NRin the respective step 720, 740 or 760, a shift reference switch data isobtained from the shift reference switch 240 in step 778 and then adetermination is made whether the shift reference switch 240 is in theon-state or the off-state in step 779. The shift reference switch dataindicates whether the shift reference switch 240 is turned on or off. Ifthe shift reference switch 240 is in the off state, the actual steppermotor pulse number data NA is retrieved from the RAM 315 in step 781.This pulse number data NA corresponds one to one to the actual rotaryposition of the stepper motor 110 unless there is any electric noise.If, in the step 779, the shift reference switch 240 is in on state, thepulse number data NA is given a zero value in step 780. The shiftreference switch 240 is so designed as to be turned on when the sleeve162 assumes a position corresponding to the largest reduction ratio.This results in the rotary position of the stepper motor 110 alwayscorresponding to the largest reduction ratio position whenever the shiftreference switch 240 is turned on. Because the actual pulse number dataNA is given a zero value whenever the shift reference switch 240 isturned on, the pulse number data NA can correspond accurately to theactual rotary position of the stepper motor 110 should there occurr asignal distortion due to electric noise. Consequently, the signaldistortion due to the accumulation of noise is eliminated. Subsequently,in step 783 (see FIG. 9(b)), the actual pulse number data NA is comparedwith the retrieved desired pulse number data ND, NL or NR.

Referring to FIG. 9(b), if the actual pulse number data NA is equal tothe desired pulse number data ND, NL or NR as the result of step 783, adetermination is made whether the desired pulse number ND, NL or NR iszero in step 785. In the case the desired pulse number ND, NL or NR isnot zero when the reduction ratio is not the largest, the same steppermotor actuating signals (described hereinafter) as provided for in thepreceding routine are sent out in step 811 before the program returns toSTART. If the desired pulse number ND, NL or NR is zero in the step 785,the shift reference switch data is obtained from the shift referenceswitch 240 in step 713, and a determination is made whether the shiftreference switch 240 is in the on state or the off state in step 715. Ifthe shift reference switch 240 is in the on state, the actual pulsenumber data NA is given a zero value in step 717, a stepper motor timervalue T which will be described later is given zero in step 718, andthen the same stepper motor actuating signals as those of the precedingroutine which correspond to the zero pulse number are sent out in step811. If, in step 715, the shift reference switch 240 is in the offstate, the execution of the steps following the step 801, which will bedescribed later, begins.

If, in the step 783, the actual pulse number NA is smaller than thedesired pulse number ND, NL or NR, the stepper motor 110 needs to beactuated toward where the pulse number increases. First, a determinationis made whether the timer value T is negative inclusive zero in step787. If the timer value T is positive, then the timer value T isdecreased by a predetermined value ΔT in step 789, and then the samestepper motor actuating signals as those of the preceding routine aresent out in step 811 before the program returns to START. This step 789is repeated until the timer value T becomes zero or negative. When thetimer value T becomes zero or negative after elapse of a predeterminedperiod of time, then the stepper motor actuating signals for the steppermotor 110 are moved in the upshift direction by one stage in step 791 asdescribed later. Then, the timer value T is given a predeterminedpositive value T1 in step 793; the stepper motor pulse number NA isincreased by 1 in step 795, and the stepper motor actuating signalswhich have been moved by one state in the upshift direction are sent outin step 811 before the program returns to START. This causes the steppermotor 110 to rotate toward the upshift direction by one unit.

If, in step 783, the present stepper motor pulse number NA is greaterthan the desired pulse number ND, NL or NR, a determination is madewhether the timer value T is zero or negative in step 801. If the timervalue T is positive, the timer value T is decreased by the predeterminedvalue ΔT (in step 803), and the same stepper motor actuating signals asthose of the preceding routine are sent out in step 811 before theprogram returns to START. After repeating this sequence of operations,the timer value T becomes zero or negative after elapse of apredetermined period of time because the decrement of the timer T by thepredetermined value ΔT is repeated. When the timer value T becomes zeroor negative, the stepper motor actuating signals are moved toward adownshift direction by one stage in step 805. Then the timer value T isgiven the predetermined positive value T1 in step 807; the stepper motorpulse number data NA is decreased by 1 in step 809, and the steppermotor actuating signals having been moved in the downshift direction aresent out (in step 811) before the program returns to START. This causesthe stepper motor 110 to rotate in the downshift direction by one unit.

Referring to FIGS. 12 to 14 and particularly to FIGS. 13 and 14, thestepper motor actuating signals will now be described. The stepper motor110 is connected with four output lead lines 317a, 317b, 317c, and 317d(see FIG. 4) having thereon respective signals which may vary in fourmodes A˜D, and the stepper motor 110 rotates in the upshift direction(the direction denoted by an arrow X as shown in FIGS. 3 and 4) if theactuating signals are moved in the sequence of A→B→C→D→A, and thestepper motor 110 rotates in the reverse or downshift direction if theactuating signals are moved in the sequence of D→C→B→A→D. Referring toFIG. 13 which illustrates the content of the bits corresponding to themode A of the actuating signals, the digit "1" is written in bitposition 0, the digit "1" in bit position 1, the digit "0" in bitposition 2, and the digit "0" in bit position 3. The bit positions 0, 1,2, 3 correspond to the signals to be applied to the respective leads317a, 317c, 317b and 317d. If the digit is "1" in a particular bitposition, a signal voltage having a high level is applied to the leadoperatively associated with the particular bit position. If the digit ina particular bit position is "0", a signal voltage having a low level isapplied to the corresponding lead. Consequently, when the stepper motor110 is to be rotated in the upshift direction, the bits are rotated tothe right, i.e., the digits are moved one place to the left. When thestepper motor 110 is to be rotated one step in the downshift direction,the bits are rotated to the left, i.e., the digits are moved one placeto the right.

The variation of the signals on the output lead lines 317a, 317c, 317b,and 317d upon upshifting is illustrated in FIG. 14. In FIG. 14, theperiod of time during which each of modes A, B, C and D stays constant,agrees with the timer value T1 which has been obtained in the step 793or 807.

As described above, the stepper motor actuating signals are moved to theleft or in the upshift direction in step 791 when the actual pulsenumber, i.e., the actual reduction ratio, is smaller than the desiredpulse number, i.e., the desired optimum reduction ratio, thus serving asactuating signals for rotating the stepper motor 110 in the upshiftdirection. In the reverse case, when the actual reduction ratio islarger than the desired optimum reduction ratio, the stepper motoractuating signals are moved to the right or in the downshift directionin step 805, thus serving as actuating signals for rotating the steppermotor 110 in the downshift direction. When the actual reduction ratioagrees with the desired optimum reduction ratio, the actuating signalsare not moved to the left nor right, and the same actuating signals asthose of the preceding routine are sent out. In this case, the steppermotor 110 will not rotate, thus maintaining the reduction ratioconstant.

If, in the previously described step 711 shown in FIG. 9(a), the R rangeis not selected, i.e., if the P range or N range is selected, theexecution of the step 713 and its following steps begins. The shiftreference switch data is obtained from the shift reference switch 240 instep 713 and if the shift reference switch 240 is in the on state, theactual pulse number NA is given a zero value in step 717 and the steppermotor timer value T is given a zero value in step 718. Then, the sameactuating signals as those of the preceding routine are sent out in step811 before the program returns to START. If the shift reference switch240 is in the off state, the steps following the step 801 are executedwhich have been described. That is, the stepper motor 110 is rotated inthe downshift direction. Accordingly, the largest reduction ratio ismaintained when the shift position is in the P or N range.

Hereinafter, a description is made as to how the desired optimumreduction ratio is determined.

Referring to FIGS. 15-19, a description will now be given of how thedesired optimum reduction ratio is determined to satisfy the minimumfuel consumption rate curve during operation in the D range.

Referring to FIG. 15, the engine performance curve is shown. In FIG. 15,engine revolution speed is expressed on the axis of abscissas and enginetorque on the axis of ordinates and there are shown engine torque vs.,engine revolution speed characteristic curves, each for a throttleopening degree (each curve being accompanied by a throttle openingdegree) and there are also shown isofuel consumption rate curves FC1˜FC8(fuel consumption rate reducing in this numerical order). In FIG. 15,the minimum fuel consumption rate curve is denoted by the character Gand the most efficient operational state is obtained if the engine isoperated on this curve G. In order to control the continuously variabletransmission so as to operate the engine along the minimum fuelconsumption rate curve G, the pulse number data ND for the stepper motor110 are determined in the following manner. If the minimum fuelconsumption rate curve G is expressed in terms of throttle openingdegree and engine revolution speed, the result may be expressed in FIG.16. As will be understood, a single engine revolution speed is given forany throttle opening degree. For example, the engine revolution speed3000 rpm is given for the throttle opening degree 40°. As shown in FIG.16, the minimum engine revolution speed 1000 rpm is given for lowthrottle opening degrees (smaller than about 20 degrees) since the drivesystem of the continuously variable transmission would exhibit resonancewith the engine vibration if the lock-up clutch is engaged with theengine revolution speeds below this minimum engine revolution speed.Assuming engine revolution speed is N and vehicle speed V, then thereduction ratio S is given by the equation:

    S=(N/V)·k

where, k denotes a constant determined by the final reduction ratio andthe radius of the tire. It will now be understood from the aboveequation and FIG. 16 that the desired optimum reduction ratio isdetermined by the vehicle speed V and the target engine revolution speedN which satisfies a predetermined relationship with the throttle openingdegrees, i.e., engine load, as shown in FIG. 16. If the relationshipshown in FIG. 16 is expressed in terms of vehicle speed rather than theengine revolution speed, the result may be expressed as shown in FIG.17. Even with the same engine revolution speed, the vehicle speeddiffers from reduction ratio to reduction ratio and this fact isexpressed in terms of a range of vehicle speed as shown in FIG. 17. Linela denotes the variation upon selecting the largest reduction ratio(reduction ratio a), and line lc denotes the variation upon selectingthe smallest reduction ratio (reduction ratio c), where line lb denotesthe variation upon selecting an intermediate reduction ratio b. Forexample, the vehicle can run at vehicle speeds from 25 km/h to 77 km/hwith the throttle opening degree 40 while the reduction ratio decreases.The reduction ratio remains at a below 25 km/h and at c above 77 km/hwith the throttle opening degree 40. A predetermined relationship existsbetween the position of the sleeve 162 of the shift operating mechanism112 and a reduction ratio. This means that a predetermined relationshipexists between the stepper motor pulse number applied to the steppermotor 110 (i.e., rotary position of the stepper motor 110) and thereduction ratio as shown in FIG. 18. Thus, the reduction ratios (a or bor c) shown in FIG. 17 can be converted into respective pulse numbersusing the graph shown in FIG. 18. The result of this conversion isillustrated in FIG. 19. Also shown in FIG. 19 are the lock-up on andlock-up off vehicle speed lines shown in FIG. 8 from which it will beunderstood that the lock-up on and lock-up off vehicle speed lines aredisposed on the lower vehicle speed side of the line a with the largestreduction ratio a.

Control of the continuously variable transmission with the shift patternillustrated in FIG. 19 is as follows. Upon moving off from a standstill,the continuously variable transmission is maintained at the largestreduction ratio and the torque converter 12 is held in the non lock-upstate. Therefore, a traction force strong enough for moving the vehicleoff from the standstill is given. When the vehicle speed exceeds thelock-up on line, the lock-up clutch 10 of the torque converter 12 (seeFIG. 1) engages, thus putting the torque converter 12 in the lock-upstate. When the vehicle speed exceeds the line la as a result of anincrease in the vehicle speed, the reduction ratio continuously variesbetween the reduction ratio and the reduction ratio c in such a manneras to satisfy the relationship denoted by the minimum fuel consumptionrate curve G shown in FIG. 15. For example, if the throttle openingdegree is increased from a state where the vehicle is running at aconstant vehicle speed with a constant throttle opening degree in anoperating range falling between lines la and lc, the desired enginerevolution speed changes and the desired pulse number changes with thechange in the desired revolution speed as determined by the relationshipillustrated in FIG. 16. The stepper motor 110 rotates to a new rotaryposition in response to the new desired stepper motor pulse number,establishing a new reduction ratio, thus allowing the actual enginerevolution speed to agree with the new target engine revolution speed.The engine is controlled to operate along with the minimum fuelconsumption rate curve G of the engine since, as described before, thestepper motor pulse number is determined to satisfy the minimum fuelconsumption rate curve G of the engine. In this manner, the reductionratio is controlled by controlling the stepper motor pulse number sinceeach reduction ratio corresponds uniquely to a single stepper motorpulse number.

From the description above, it will be understood that the desiredoptimum reduction ratio is determined by the vehicle speed and thedesired engine revolution speed which satisfies the predetermonedrelationship with the engine load.

In the embodiment described above, the control is based on the enginethrottle opening degree, but it is also possible to carry out a similarcontrol based on the intake manifold vacuum or the fuel flow rate. FIGS.20 and 21 illustrate the minimum fuel consumption rate curves G for thelatter two cases, respectively.

The above description has focused mainly on the shift pattern to befollowed upon selecting the D range, but all that is necessary foroperation in the L range or R range is to give data relating todifferent shift patterns from that in D range. For example, for the samethrottle opening degree, a shift pattern for the L range is designed togive a larger reduction ratio as compared to the reduction ratio whichis given by the shift pattern for the D range for the purpose ofenhancing acceleration performance and ensuring adequate engine brakingperformance at zero throttle opening degree. In a shift pattern for theL range, a reduction ratio larger than the reduction ratio given by theshift pattern for the D range is given for the same throttle openingdegree. These shift patterns can be accomplished simply by inputtingappropriate predetermined pulse data. A more detailed explanation of theoperation in the L and R ranges is omitted since the basic actionscarried out to effect the control are the same as in the D range.

Next, a brief explanation will be given as to the engine coolanttemperature sensor 306 and the brake sensor 307 shown in FIG. 4.

The engine coolant temperature sensor 306 is switched "on" when theengine coolant temperature is below a predetermined value (for example,60° C.). When the engine coolant temperature sensor 306 is in the "on"state, the shift pattern for the D range is switched in response to thissignal to a shift pattern having larger reduction ratios. Thiseliminates irregular running of the engine and engine power shortagewhich otherwise would take place upon start-up of a cold engine.

The brake sensor 307 is switched "on" when the foot brake is actuated.If the brake sensor 307 is in the "on" state and at the same time thethrottle opening degree is zero, the shift pattern for the D range isswitched to a shift pattern giving larger reduction ratios. This ensuresstrong engine braking upon depressing the brake when operating in the Drange.

A second embodiment is described in connection with FIGS. 22(a) and22(b). This embodiment uses a drive pulley revolution speed as a basefor controlling a temporal release of the engagement of a lock-upclutch.

Steps from 501 to 1707 of a lock-up solenoid control routine 500' areidentical to the counterparts of the first embodiment shown in FIGS.5(a) and 5(b). It is also the same to retrieve a D range shift patternand a L range shift pattern (steps 2720 and 2740), but it is differentin that a ROM 314 (see FIG. 4) has stored therein desired revolutionspeed of a drive pulley 24 as data and a desired drive pulley revoutionspeed NPD is retrieved. Then, in a step 2781, an actual drive pulleyrevolution speed NPA is obtained from a drive pulley revolution speedsensor 1301 shown diagrammatically in FIG. 1. A difference ΔNP betweenthe desired pulley revolution speed NPD and the actual drive pulleyrevolution speed NPA is computed in step 2783 by subtracting NPA fromNPD. A determination is made whether NP is greater than or equal to apositive value ΔNP1 or not in step 2785. If ΔNP is less than ΔNP1 (i.e.,in the case wherein a rapid down shift is not needed), a determinationis made whether a timer T3 is zero or not in step 2787. The fact thatthe timer T3 is zero represents a state that the engagement of thelock-up clutch is not released, while the fact that the timer T3 is notzero represents a state that the engagement of the lock-up clutch isreleased temporarily. If the timer T3 is zero in step 2787, the programgoes to step 509 and its following steps. The step 509 and its followingsteps are the same as the counterparts in FIGS. 5(a) and 5(b). If, instep 2785, ΔNP is greater than or equal to ΔNP1 (i.e., in the case arapid down shift is needed) and, at the same time, T3 is not equal tozero (i.e., in the case the engagement of the lock-up clutch isreleased), the timer T3 is increased by a predetermined small value ΔT3in step 2789. A determination is made whether this timer T3 is greaterthan or equal to a predetermined lock-up temporal release time t3 instep 2791, and if T3 is less than t3 (i.e., the predetermined lock-uptemporal release time has not been expired), the program goes to step567 to deactuate the lock-up solenoid 200 to disengage the lock-upclutch. If, in step 2791, T3 is greater than or equal to t3 (i.e., inthe case the lock-up temporal release time has expired), the timer T3 isgiven zero value in step 2793. As a result, in the next routine, theprogram goes to step 509 from the step 2787.

With the execution of a series of steps 2781 to 2791, the engagement ofthe lock-up clutch is released from the predetermined time t3 after thedetermination is made that the rapid down shift is needed. Thus, thisembodiment provides for similar operation and effect to those of thepreviously described first embodiment. Because the release of theengagement of the lock-up clutch is assured for the predetermined timet3, a strong acceleration is provided.

A third embodiment is described in connection with FIGS. 23(a ) and23(b). This embodiment uses a reduction ratio as a base for controllingthe temporal release of the engagement of the lock-up clutch.

Steps 501 to 1707 of a lock-up solenoid control routine 500" areidentical to the countertparts of the first embodiment shown in FIGS.5(a) and 5(b). It is also the same to retrieve a D range shift patternand a L range shift pattern (steps 3720 and 3740), but it is differentin that a ROM 314 has stored therein desired reduction ratio as data anda desired reduction ratio iD is retrieved. Then, in a step 3781, anactual drive pulley revolution speed NPA is obtained from a drive pulleyrevolution speed sensor 1301 (see FIG. 1). An actual reduction ratio iAis obtained from the actual drive pulley revolution speed NPA andvehicle speed V by computing the equation iA=NPA/(k·V) (k: constant) instep 3782. A difference ΔiP between iD and iA is computed in step 3783by subtracting iA from iD. A determination is made whether ΔiP isgreater than or equal to a predetermined positive value ΔiP1 or not instep 3785. If ΔiP is less than ΔiP1 (i.e., in the case a rapid downshifting is not needed), a determination is made whether a timer T4 iszero or not in step 3787. The fact that the timer T4 is zero representsa state that the engagement of the lock-up clutch is temporarilyreleased, while the fact that the timer T4 is not zero represents astate that the engagement of the lock-up clutch is not released. If thetimer T4 is not zero, the timer T4 is given a predetermined value t4 instep 3788 and then the program goes to step 509. The step 509 and itsfollowing steps are the same as the counterparts in FIGS. 5(a) and (b).If, in step 3785, ΔiP is greater or equal to ΔiP1 (i.e., in the case arapid down shift is needed), the timer T4 is given zero in step 3786 andthe program goes to a step 567 to deactuate the lock-up solenoid 200.If, at step 3787, T4 is equal to zero (i.e., the engagement of thelock-up clutch is temporarily released), the timer T4 is increased by apredetermined small value ΔT4 in step 3789, a determination is madewhether the timer T4 is greater than or equal to a predetermined lock-uptemporal release time t4 or not in step 3791, and if T4 is less than t4(i.e., in the case the lock-up temporal release time has not yetexpired), the program goes to step 567 to deactuate the lock-up solenoid200. If, in step 3791, T4 is greater than or equal to t4 (i.e., thelock-up temporal release time has expired), the program goes to step509.

With the execution of a series of steps 3781 to 3791, the engagement ofthe lock-up clutch is released for the predetermined time t4 after thedetermination is made that the rapid down shift is needed. Thus, thisembodiment provides for similar operation and effect to those of thepreviously described first and second embodiments.

As described above, according to the present invention, the engagementof a lock-up clutch is released when a rapid acceleration is needed(i.e., when a rapid down shift is needed), allowing a hydrodynamicdevice to slip, thus allowing the engine to increase its speed forproducing increased power output. In the case of a torque converter, amultiplied torque is obtained. As a result, the response to shiftingdemand has been enhanced and drive feel as well as safety have beenimproved owing to strong acceleration.

What is claimed is:
 1. A method for controlling an automotive vehicleincluding an engine and a continuously variable transmission whichincludes a hydrodynamic device having an input element drivinglyconnected to the engine and an output element, a drive pulley drivinglyconnected to the output element, a driven pulley, the V-belt runningover the drive and driven pulleys, and a lock-up clutch engageable tomechanically connect the input element with the output element, thetransmission being shiftable between different reduction ratios andhaving a shift motor which is movable to different positions foreffecting shifting to different reduction ratios, the methodcomprising:detecting a demand for a rapid acceleration wherein a rapiddown shift is needed and generating a rapid down shift need indicativesignal; and disengaging the lock-up clutch in response to presence ofsaid rapid down shift need indicative signal so as to allow the engineto increase its revolution speed during the down shift.
 2. A method asclaimed in claim 1, wherein said rapid down shift need indicative signalgenerating step includes:generating an actual position indicative signalindicative of an actual position of the shift motor; generating adesired position indicative signal representing a desired position ofthe shift motor; computing a difference between said actual positionindicative signal and said desired position indicative signal andgenerating a difference indicative signal representing said difference;and generating said rapid down shift need indicative signal when saiddifference indicative signal is greater than a predetermined value.
 3. Amethod as claimed in claim 1, wherein said rapid down shift needindicative signal generating step includes:generating an actual drivepulley revolution speed indicative signal representing an actualrevolution speed of the drive pulley; generating a desired drive pulleyrevolution speed indicative signal representing a desired revolutionspeed of the drive pulley; computing a difference between said desireddrive pulley revolution speed indicative signal and said actual drivepulley revolution speed indicative signal and generating a differenceindicative signal representing said difference; and generating saidrapid down shift need indicative signal when said difference indicativesignal is greater than a predetermined value.
 4. A method as claimed inclaim 1, wherein said rapid down shift need indicative signal generatingstep includes:generating an actual reduction ratio indicative signalrepresenting an actual reduction ratio in the transmission; generating adesired reduction ratio indicative signal representing a desiredreduction ratio; computing a difference between said desired reductionratio indicative signal and said actual reduction ratio indicativesignal and generating a difference indicative signal representing saiddifference; and generating said rapid down shift need indicative signalwhen said difference indicative signal is greater than a predeterminedvalue.
 5. A method as claimed in claim 1, wherein said lock-up clutchdisengaging step is maintained for a predetermined time beginning withthe detecting of said demand.
 6. An apparatus for controlling acontinuously variable transmission of an automotive vehicle having aninternal combustion engine, the transmission including a hydrodynamicdevice having an input element and an output element, a drive pulley, adriven pulley, a V-belt running over the drive and driven pulleys, and alock-up clutch engageable to mechanically connect the input element tosaid output element, the transmission being shiftable between differentreduction ratios, the apparatus comprising:means including a shift motormovable to different positions for shifting the transmission todifferent reduction ratios corresponding uniquely to said positions;means for detecting a demand for a rapid acceleration wherein a rapiddown shift is needed and generating a rapid down shift need indicativesignal; and means for disengaging the lock-up clutch in response topresence of said rapid down shift need indicative signal to allow theengine to increase its revolution speed during the down shift.
 7. Anapparatus for controlling a continuously variable transmission of anautomotive vehicle having an internal combustion engine, thetransmission including a hydrodynamic device having an input element andan output element, a drive pulley, a driven pulley, a V-belt runningover the drive and driven pulley, and a lock-up clutch engageable tomechanically connect the input element to the output element, thetransmission being shiftable between reduction ratios, the apparatuscomprising:means for detecting a vehicle speed of the automotive vehicleand generating a vehicle speed indicative signal; means for detecting anengine load on the internal combustion engine; means for retrieving adesired optimum reduction ratio for the detected vehicle speed andengine load and generating a desired optimum reduction ratio indicativesignal; means for generating an actual reduction ratio indicative signalrepresenting an actual reduction ratio of the continuously variabletransmission; means for computing a deviation of the desired optimumreduction ratio indicative signal from the actual reduction ratioindicative signal and generating a deviation indicative signalrepresenting said deviation; means for comparing said deviation with apredetermined value and generating a rapid down shift need indicativesignal when said deviation is greater than said predetermined value; ashift motor rotatable between a plurality of rotary positions thereof;means operatively connected to said shift motor to be actuated therebyfor shifting the transmission to a reduction ratio corresponding to oneof the plurality of rotary positions of said shift motor assumed by saidshift motor; means responsive to said vehicle speed indicative signalfor engaging the lock-up clutch; and means for disengaging the lock-upclutch temporarily in response to said rapid down shift need indicativesignal so as to allow the engine to increase its revolution speed duringthe rapid down shift.
 8. An apparatus as claimed in claim 7, whereinsaid desired optimum reduction ratio indicative signal and said actualreduction ratio indicative signal are expressed in terms of the numberof pulses supplied to said shift motor.
 9. An apparatus as claimed inclaim 7, wherein said desired optimum reduction ratio indicative signaland said actual reduction ratio indicative signal are expressed in termsof the revolution speed of the drive pulley.
 10. An apparatus as claimedin claim 7, wherein said desired optimum reduction ratio indicativesignal and said actual reduction ratio indicative signal are expressedin terms of the reduction ratio of the transmission.
 11. A method forcontrolling an automotive vehicle including an engine and a continuouslyvariable transmission which includes a torque converter having an inputelement drivingly connected to the engine and an output element, a drivepulley drivingly connected to the output element, a driven pulley, aV-belt running over the drive and driven pulleys, and a lock-up clutchengageable to mechanically connect the input element with the outputelement, the transmission being shiftable between different reductionratios continuously, the method comprising:a first step of detecting anoperating condition of the automotive vehicle and generating anoperating condition indicative signal; a second step of retreiving adesired value indicative of a desired reduction ratio in response tosaid operating condition indicative signal and generating a desiredvalue indicative signal; a third step of obtaining an actual valueindicative of an actual reduction ratio in the continuously variabletransmission and generating an actual value indicative signal; a fourthstep of computing a deviation of said desired value indicative signalfrom said actual value indicative signal and generating a deviationindicative signal; a fifth step of comparing said deviation indicativesignal with a predetermined reference; a sixth step of disengaging thelock-up clutch; and repeating a routine including said first, second,third, fourth, fifth and sixth steps when said deviation indicativesignal satisfies a predetermined relationship with said predeterminedreference.
 12. A method for controlling an automotive vehicle includingan engine and a continuously variable transmission which includes atorque converter having an input element drivingly connected to theengine and an output element, a drive pulley drivingly connected to theoutput element, a driven pulley, a V-belt running over the drive anddriven pulleys, and a lock-up clutch engageable to mechanically connectthe input element with the output element, the transmision beingshiftable between different reduction ratios continuously, the methodcomprising:a first step of detecting an operating condition of theautomotive vehicle and generating an operating condition indicativesignal; a second step of retrieving a desired value indicative of adesired reduction ratio in response to said operating conditionindicative signal and generating a desired value indicative signal; athird step of obtaining an actual value indicative of an actualreduction ratio in the continuously variable transmission and generatingan actual value indicative signal; a fourth step of computing adeviation of said desired value indicative signal from said actual valueindicative signal and generating a deviation indicative signal; a fifthstep of comparing said deviation indicative signal with a predeterminedreference; a sixth step of increasing a timer by a predetermined timervalue; a seventh step of comparing the timer with a predetermined elapsetime; an eighth step of disengaging the lock-up clutch; a ninth step ofsetting the timer to zero; a tenth step of comparing the timer withzero; and repeating a first routine including said first, second, third,fourth, fifth, sixth, seventh and eighth steps when said deviationindicative signal satisfies a predetermined relationship with saidpredetermined reference and said timer is less than said predeterminedelapse time; repeating a second routine including said first, second,third, fouth, fifth, sixth, seventh, ninth and eighth steps when saiddeviation indicative signal satisfies said predetermined relationshipwith said predetermined reference and said timer is not less than saidpredetermined elapse time; and repeating a third routine including saidfirst, second, third, fourth, fifth, tenth, sixth, seventh steps whensaid timer is less than said predetermined elapse time even when saiddeviation indicative signal fails to satisfy said predeterminedrelationship with said predetermined reference.